Objective Comments and Analysis - All Science, No Politics
Contributions by Richard James and Rick Thoman

Monday, April 20, 2015

Suddenly Snowless

Chinook winds and warmth in Fairbanks eliminated the remains of the valley-level winter snowpack on Saturday, with the official daily snow depth measurement dropping from 5" to only a trace in a single day. The last time this much snow was removed in one day was in April 2004, and for context the record 1-day decrease is 8" on several occasions (most recently April 24, 1993). Snow removal of this efficiency is accomplished by sustained winds that allow for large heat transfer to the snow surface.

The April 18 end of the winter snowpack is 5 days earlier than the long-term median date and just barely falls within the earliest tercile of melt-out dates; so it is earlier than usual but not very much so. The earliest melt-out in Fairbanks was on April 3, 1970, although in that year snow returned the very next day; there was a more abiding melt-out on April 6, 1965.

Is this the last of any snow cover in Fairbanks until autumn? Not necessarily. Almost half of all years see some accumulating snow after the first melt-out date, although only 20% of years see an inch or more of snow depth reported. More than 1 inch of reported snow depth is rare after melt-out, so any snow that accumulates from here is very likely to be light and/or short-lived.

It's interesting to observe that the chinook warmth has also done a number on river ice in the area, and the tripod at Nenana is on its last legs (see below). If the clock is tripped today, it will tie the all-time record for earliest breakup date (1940, 1998). It's a remarkable turn-around from the cold spring and record late breakup of 2013.

3 comments:

As Brian mentions in his blog, part of the reason for the sudden decrease from 5" to trace is partly due the definition used: at least 50% coverage in the observation area. If 45% of the area had 5", it would still count as trace.

However, there has been a fast melt the last few days for sure. The low temperature being near freezing has helped greatly too.

Great point Eric (and Brian). I neglected to consider that. Still, a rapid melt-out. The low temperature of 40F on Sunday was the second earliest on record for such a high minimum temperature. The long-term median for first such warm night is May 12.